The present invention relates to a pipe comprising an ethylene base polymer, more specifically to a pipe comprising an ethylene base polymer which is excellent in a creep characteristic (durability), a rigidity and an impact resistance and which is suited to uses for large diameter pipes receiving a high internal pressure.
In recent years, ethylene base polymers are increased in use in the pipe field because of characteristics thereof such as an excellent impact resistance. The most important characteristic required to the pipes of the ethylene base polymers is to have such an excellent durability that they are not broken even under pressure exerted by gas and city water over a long period of time. However, conventional polyethylene pipes have a problem on any of a rigidity, a long-term durability and an impact resistance, and the existing situation is that particularly when they are used for large diameter pipes receiving a high internal pressure, they can not be used because of an unsatisfactory creep characteristic and are specialized in use for branch pipes having a small diameter.
Then, various proposals have been made in order to apply them to uses receiving a high internal pressure. A polyethylene resin composition for a pipe is proposed in, for example, Japanese Patent Application Laid-Open No. 57638/1986. However, this composition has the defect that it has a high density and therefore is liable to be susceptible to brittle fracture in a creep test. Further, a polyethylene-made pipe having excellent mechanical characteristics is proposed in Japanese Patent Application Laid-Open No. 301933/1996, but this has the problem that it is inferior in fluidity.
Further, a production process in which annealing conditions are specified in order to obtain a pipe having an excellent low temperature toughness is proposed in Japanese Patent Application Laid-Open No. 44219/1998, but it is not preferred in terms of heat balance to heat again an article once cooled to subject to annealing treatment and further cool it down, and a long production line is required in actual molding.
The present invention has been made under such circumstances, and an object thereof is to provide a pipe comprising an ethylene base polymer which is excellent in a durability, a rigidity and an impact resistance and which is suited to uses for a large diameter pipe receiving a high internal pressure.
The present inventors have repeated intensive researches in order to achieve the object described above. As a result thereof, they have found that a pipe made of an ethylene base polymer satisfying fixed ranges in four properties of (1) residual stress, (2) crystallinity, (3) gradient of the crystallinity against a thickness of the pipe and (4) a thickness of the pipe has desired physical properties and is a pipe meeting the above object. The present invention has been completed based on such knowledge.
That is, the present invention provides a pipe comprising an ethylene base polymer comprising ethylene and xcex1-olefin having 3 to 20 carbon atoms, wherein (A) a maximum value of the residual stress is 0.13 MPa or less; (B) the crystallinity is 0.630 to 0.850; (C) a gradient of the crystallinity against a thickness of the pipe is 0.465 or less; and (D) the pipe has a thickness of 5 to 50 mm.